Portable electronic devices are incorporating load switches to implement power management functions. In particular, there is a need in portable electronic devices to extend battery life while maintaining performance level. Load switches are used in portable electronic devices to enable the system to make power management decisions based on the peripherals or sub-circuits currently being used. When incorporated in an electronic device, a load switch is controlled by the electronic device to connect or disconnect a voltage rail to a specific load. The load switch provides a means to power a load when it is in demand so as to allow the electronic device to maximize performance.
A load switch circuit is typically implemented using a switching device controlled by an on-off control signal. The switching device is typically a MOSFET (metal-oxide-semiconductor field-effect-transistor) device that passes the supply voltage to a specific load when the transistor is turned on by the control signal. Load switch circuit can be configured for high-side switching or low-side switching. FIG. 1, which includes FIGS. 1(a) to 1(c), illustrates examples of conventional load switch circuits. More specifically FIG. 1(a) illustrates a load switch circuit configured for high-side switching. The load switch circuit includes a load switch LS1 to connect a load 10 to a supply voltage 12, which is a battery voltage VBAT in the present example. For high-side switching, load switch LS1 is typically implemented as a PMOS transistor.
FIG. 1(b) illustrates a load switch circuit configured for low-side switching. The load switch circuit includes a load switch LS2 to connect a load 10 to a supply voltage 12, which is a battery voltage VBAT in the present example. For low-side switching, load switch LS2 is typically implemented as an NMOS transistor, as shown in FIG. 1(c). Referring to FIG. 1(c), a low-side load switch is implemented using an NMOS transistor M10 configured to couple the supply voltage 12 to the load 10, represented as a resistor RLoad. The transistor M10 is controlled by a control voltage Vin (node 14) which is typically a slow ramp voltage signal. A slow ramp control voltage is used to turn on the load switch slowly so that the load 10 does not experience a current surge.
However, as the gate voltage of the load switch transistor M10 ramps up, the transistor may be biased in the saturation region or linear mode for a long time. When biased in the saturation region, the transistor M10 operates at high drain current and high drain-to-source voltage. When the transistor M10 dwells in the saturation region and operating at high current and high voltage for a long time, the load switch transistor can heat up. Excessive heating at the load switch may result in thermal runaway during the control voltage ramp up time, resulting in permanent device failure.